1. Field of the Invention
This invention relates to a lubrication system for a V-type overhead camshaft engine, and, more particularly, to a lubrication system comprising various oil passages formed in a cylinder head of a V-type overhead camshaft engine which is equipped with a hydraulic valve lash adjuster for each valve and a variable valve timing mechanism.
2. Description of Related Art
A double overhead camshaft engine has a pair of overhead camshafts, namely an intake camshaft and an exhaust camshaft, each equipped with cams, and which are coupled to each other by a drive camshaft gear and a driven camshaft gear in mesh with each other. One of the camshafts is equipped with a camshaft sprocket, or a camshaft pulley, which is connected or coupled to the crankshaft by a timing belt forming a drive connection between the engine output shaft and the camshaft pulley.
Coupling the overhead camshafts with the drive and driven camshaft gears allows the overhead camshafts to be located close to each other, so that the intake and exhaust valves can be arranged at a small acute angle relative to each other with respect to a combustion chamber. This results in a small size of engine body and a simple combustion chamber structure with increased fuel combustion efficiency.
Typically, an engine valve drive mechanism or system is equipped with a variable valve timing mechanism for changing the timing at which the intake and exhaust valves are opened and closed by the camshafts. The variable valve timing mechanism cooperates with a camshaft to change the rotational phase of the camshaft so as to retard or advance the valve timing of one of the intake and exhaust valves relative to the other according to engine operating conditions. Such an engine valve drive mechanism is known, for example, from Japanese Unexamined Utility Model Publication No. 62-57711.
The variable valve mechanism described in the above publication is one which is actuated by a hydraulic control system and controlled by an electromagnetic actuator disposed in an oil outflow portion at the end of the camshaft equipped with the variable valve timing mechanism.
It is also known to provide a valve lash adjuster, such as a hydraulic valve lash adjuster (HLA), to perform automatic adjustment of valve clearance. Oil is supplied to the valve lash adjuster from an oil passage in the cylinder head for lubricating camshaft journal bearings.
In order for the double overhead camshaft engine, to supply working oil to the variable valve timing mechanism and the valve lash adjuster as well as to bearings for the intake and exhaust overhead camshafts, the engine body must be provided with oil passages. However, in a double overhead camshaft engine, which is compact in size and has a narrow space between the intake and exhaust overhead camshafts, it is difficult to provide a plurality of oil passages without interfering with the valves and spark plugs. Further, providing a number of oil passages is not favorable either for the valve lash adjusters which require high pressure oil to work with accuracy and high efficiency, or for the engine body which requires a high rigidity of structure.
In an engine equipped with a variable valve timing mechanism and a valve lash adjuster as described above, if an oil supplying system to both the variable valve timing mechanism and the valve lash adjuster is provided with an orifice, for regulating the quantity and pressure of oil supplied to the cylinder head for the purpose of maintaining a necessary oil pressure to the cylinder head and reducing an unnecessary flow of oil in the cylinder head, disposed between the cylinder block and cylinder head, it is difficult to maintain both the variable valve timing mechanism and the valve lash adjuster under well controlled operation.
More particularly, for the purpose of maintaining lubrication oil at a necessary pressure in the cylinder block and balancing the quantities of oil delivered into the cylinder block and the cylinder head, the oil available for flow into the cylinder head is somewhat restricted. Also, when the working pressure of oil is low due to an insufficient supply of oil to the valve lash adjuster while the engine operates at lower speeds, such as during idling, and generates lower oil pressure, while the oil temperature is high, or when starting the engine after a long period left not operated, etc., it takes time to discharge air within the valve lash adjuster, leading to the generation of an anomalous sound.
To prevent the generation of such anomalous sound, the air within the valve lash adjuster may be forced out in a short time by introducing oil at high pressure into the valve lash adjuster. However, the high pressure oil is their also supplied, unnecessarily, to the camshaft bearing means, which do not inherently need high pressure lubrication oil, so as to cause an increase of end flow oil, and accordingly, a large capacity of oil pump is necessary.